[clang] Fix bug with -ffp-contract=fast-honor-pragmas (PR #104857)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

Changes LGTM modulo nits, but please also add a release note to `clang/docs/ReleaseNotes.rst` so users know about the fix.

https://github.com/llvm/llvm-project/pull/104857